1. Field of the Invention
The present invention relates to a photo-semiconductive electrode and to a photo-electric cell using the photo-semiconductive electrode. More specifically, the present invention relates to a wet-type photo-electric cell in which the photo-semiconductive electrode and a counter electrode contact an electrolyte, and also to the photo-semiconductive electrode which is used in this photo-electric cell.
2. Description of the Related Art
Recently, research has been actively pursued on energy utilizing solar energy, which is advantageous to the environment, as an energy source. Particularly, as a result of intensive studies of an element which directly converts solar energy into electric energy by using an inorganic semiconductor such as a silicon semiconductor or a gallium arsenide semiconductor, photo-electric cells, using a single crystal, a polycrystal or an amorphous thin film have been put to practical use. However, problems associated with these photo-electric cells are as follows. The production of these photo-electric cells that use inorganic semiconductors requires very high-level technology and large amounts of energy. These photo-electric cells cannot be described as an energy source that is advantageous to the environment because a toxic gas is used as a raw material for the production of the photo-electric cells. Further, in terms of cost, processes required to produce these photo-electric cells, including, for example, purification of raw materials, preparation of single crystals, high-speed slicing of crystals by means of multi-wire, preparation of amorphous thin films, and doping, lead to low productivity and high production costs. In addition, applications of these photo-electric cells are limited because it is difficult to set photo-electric cells using a crystalline system to a curved area having a small radius of curvature and because it is difficult to arrange these photo-electric cells in a rational manner for the purpose of increasing the light-receiving area, for example.
In order to overcome the above-mentioned problems of the photo-electric cells using inorganic semiconductors, intensive studies are being carried out on elements which exhibit photoelectromotive force using an organic semiconducting material, characterized by advantages such as diversity, low toxicity and ease of workability. For an element that exhibits photoelectromotive force to produce a photocurrent, the presence of an internal electric field is required. Several methods which produce the internal electric field in an element using an organic material are known.
1) Schottky Junction or MIS Junction
In this construction, an internal electric field produced by a metal/semiconductor junction is utilized. As the organic semiconductive material, those which utilize a merocyanine dye or a phthalocyanine pigment, have been reported (A. K. Ghosh et al., J. Appl. Phys. 49, p.5982, 1978). Problems associated with this constructionare as follows. Although these elements can produce a large open-circuit voltage, transmissivity to light of the electrode is reduced since a metallic material is used as an electrode. Further, since the oxidation resistance thereof is insufficient, the stability is insufficient and therefore the light-to-electricity conversion efficiency is undesirably reduced.
2) Heterogeneous p-n Junction Utilizing an n-type Organic Semiconductor/p-type Organic Semiconductor Junction
As the n-type organic semiconducting material, an electron-donating organic material exemplified by dyes such as Malachite Green dye or Methyl Violet dye, or by a perylene pigment is utilized, and as the p-type organic semiconducting material, that which utilizes an electron-accepting material exemplified by a merocyanine dye or a phthalocyanine pigment have been reported (C. W. Tang et al., Appl. Phys. Lett. 48, p.183, 1986). Problems associated with this construction are as follows. Since constructions of these elements are complicated, pinholes tend to occur when films are formed. As a result, characteristics such as photocurrent, open-circuit voltage and stability are inadequate. Further, in a case where thin films are formed by a deposition process, production costs are undesirably high because, several manufacturing processes are required such as the formation of amorphous thin films for photo-electric cells using inorganic semiconductors.